The overall objective of this study is to obtain information which may lead to an understanding of the mechanism by which fluoride, in particular topical fluorides, brings about caries inhibition. The effect of in vitro caries-like lesion formation in enamel on the distribution of naturally acquired fluoride and fluoride acquired from one or more topical fluoride application will be monitored. This will be done by comparing the distribution of fluoride in sound enamel, determined by an acid etch method, and that in adjacent carious enamel using a microdissection technique. Here the caries-like lesions, produced by the acid gels, will be sectioned and the lesion dissected to provide the use of Ca and P percent weight and enamel density estimations for sound enamel being used in the calculation of fluoride concentrations in caries-like lesions. The changes in histological appearance and fluoride distribution in white spot lesions after one or more topical fluoride applications will be investigated using polarized light microscopy and microdissection respectively. The fluoride distribution will be obtained from the same enamel section that has undergone histological investigation using polarized light microscopy. The effect of lesion progression on the histological appearance and fluoride distribution of white spot lesions after one or more topical fluoride applications carried out at 24 hour intervals or incremental fluoride application carried out at 3 month intervals (prior to each cariogenic challenge) will be investigated. By studying these parameters it will be possible to determine: whether the anit-caries effect of fluoride is dependent upon its direct action on the developed or developing carious (white spot) lesions; which, when and how many applications of a topical fluoride has the greatest effect on preventing lesion initiation and/or progression; whether the acid gel system would make a good screening technique for potential anticaries agents.